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GETGROUPS(2)		   Linux Programmer's Manual		  GETGROUPS(2)

       getgroups, setgroups - get/set list of supplementary group IDs

       #include <sys/types.h>
       #include <unistd.h>

       int getgroups(int size, gid_t list[]);

       #include <grp.h>

       int setgroups(size_t size, const gid_t *list);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

	   Since glibc 2.19:
	   Glibc 2.19 and earlier:

       getgroups()  returns the supplementary group IDs of the calling process
       in list.	 The argument size should be set  to  the  maximum  number  of
       items  that  can	 be  stored  in the buffer pointed to by list.	If the
       calling process is a member of more  than  size	supplementary  groups,
       then  an	 error results.	 It is unspecified whether the effective group
       ID of the calling process is included in the returned list.  (Thus,  an
       application should also call getegid(2) and add or remove the resulting

       If size is zero, list is not modified, but the total number of  supple-
       mentary	group IDs for the process is returned.	This allows the caller
       to determine the size of a dynamically allocated list to be used	 in  a
       further call to getgroups().

       setgroups()  sets  the supplementary group IDs for the calling process.
       Appropriate privileges are required (see the description of  the	 EPERM
       error, below).  The size argument specifies the number of supplementary
       group IDs in the buffer pointed to by list.

       On success, getgroups() returns the number of supplementary group  IDs.
       On error, -1 is returned, and errno is set appropriately.

       On success, setgroups() returns 0.  On error, -1 is returned, and errno
       is set appropriately.

       EFAULT list has an invalid address.

       getgroups() can additionally fail with the following error:

       EINVAL size is less than the number of supplementary group IDs, but  is
	      not zero.

       setgroups() can additionally fail with the following errors:

       EINVAL size  is	greater than NGROUPS_MAX (32 before Linux 2.6.4; 65536
	      since Linux 2.6.4).

       ENOMEM Out of memory.

       EPERM  The calling process has insufficient privilege (the caller  does
	      not  have	 the  CAP_SETGID  capability  in the user namespace in
	      which it resides).

       EPERM (since Linux 3.19)
	      The use of setgroups() is denied in this	user  namespace.   See
	      the description of /proc/[pid]/setgroups in user_namespaces(7).

       SVr4,   4.3BSD.	 The  getgroups()  function  is	 in  POSIX.1-2001  and
       POSIX.1-2008.  Since setgroups() requires privilege, it is not  covered
       by POSIX.1.

       A  process  can have up to NGROUPS_MAX supplementary group IDs in addi-
       tion to the effective group ID.	The constant NGROUPS_MAX is defined in
       <limits.h>.   The  set of supplementary group IDs is inherited from the
       parent process, and preserved across an execve(2).

       The maximum number of supplementary group IDs can be found at run  time
       using sysconf(3):

	   long ngroups_max;
	   ngroups_max = sysconf(_SC_NGROUPS_MAX);

       The  maximum return value of getgroups() cannot be larger than one more
       than this value.	 Since Linux 2.6.4, the maximum number	of  supplemen-
       tary  group  IDs is also exposed via the Linux-specific read-only file,

       The original Linux getgroups() system call supported only 16-bit	 group
       IDs.   Subsequently,  Linux  2.4 added getgroups32(), supporting 32-bit
       IDs.  The glibc getgroups() wrapper function transparently  deals  with
       the variation across kernel versions.

   C library/kernel differences
       At the kernel level, user IDs and group IDs are a per-thread attribute.
       However, POSIX requires that all threads in a process  share  the  same
       credentials.   The  NPTL	 threading  implementation  handles  the POSIX
       requirements by providing wrapper  functions  for  the  various	system
       calls  that  change  process  UIDs  and	GIDs.  These wrapper functions
       (including the one for setgroups()) employ a signal-based technique  to
       ensure  that  when  one	thread	changes	 credentials, all of the other
       threads in the process also change their credentials.  For details, see

       getgid(2),  setgid(2), getgrouplist(3), group_member(3), initgroups(3),
       capabilities(7), credentials(7)

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Linux				  2016-10-08			  GETGROUPS(2)